Lesson Explainer: Group 15 Chemistry

In this explainer, we will learn how to describe and explain the properties and economic importance of the group 15 elements.

On the right-hand side of the periodic table, as shown below, we find group 15 elements, also known as pnictogens.

Group 15 contains six elements that run from nitrogen through to moscovium down the periodic table:

  • Nitrogen, N
  • Phosphorus, P
  • Arsenic, As
  • Antimony, Sb
  • Bismuth, Bi
  • Moscovium, Mc

Example 1: Identifying a Group 15 Element from a Selection of Chemical Symbols

Which of the following is a group 15 element?

  1. Na
  2. N
  3. Ni
  4. Ne
  5. Nb

Answer

Group 15 is on the right-hand side of the periodic table, and it consists of the elements nitrogen, phosphorus, arsenic, antimony, bismuth, and moscovium. The question presents us with a number of choices in the form of chemical symbols. All of these symbols start with the letter N, potentially trying to confuse us when we consider nitrogen, the only group 15 element whose symbol begins with N.

We can determine that B is the correct answer either through factual recall or a step-by-step elimination process. Na is the symbol for sodium, Ni is the symbol for nickel, Ne is the symbol for neon, and Nb is the symbol for niobium. So, the correct answer is B.

Nitrogen and phosphorus can be found at the top of the group 15 periodic table column, and they are both nonmetals. Arsenic and antimony are metalloids, and they can be found in the periodic table underneath the elements nitrogen and phosphorus.

The fifth element belonging to group 15 is bismuth. Bismuth is a metal, but, unusually for a metal, it is not a particularly effective conductor of electricity.

Moscovium is a synthetic element that was first synthesized in 2003 by a team of scientists in Russia, and it was only recognized as an official chemical element by the IUPAC in 2015. Moscovium is an extremely radioactive element, and we have only managed to produce approximately 100 moscovium atoms during the last two decades. We will not be covering moscovium much in this explainer.

Overall, as we descend group 15, the elements become more metallic.

Example 2: Identifying a Pair of Group 15 Elements That Are Classified as Metalloids

Which of the following pairs are metalloids included in group 15 elements?

  1. Phosphorus and bismuth
  2. Bismuth and arsenic
  3. Antimony and arsenic
  4. Phosphorus and arsenic
  5. Antimony and bismuth

Answer

As we descend group 15, the properties of the elements change from nonmetals to metalloids and then to metals. The correct order of elements in group 15 from top to bottom is nitrogen, phosphorus, arsenic, antimony, bismuth, and moscovium.

Nitrogen and phosphorus are considered nonmetals, and bismuth is considered a metal. This leaves us with the elements arsenic and antimony, as both are metalloids. We can conclude, therefore, that the correct answer is C.

Nitrogen is the most abundant of group 15 elements. It can be found in compounds that make up part of Earth’s continental crust. However, nitrogen is mainly found in the air. As a primary component of the atmosphere, nitrogen makes up 78% of atmospheric air.

The other elements in this group are not found in large quantities in Earth’s crust and decrease in abundance as you descend the group. The approximate percentage abundance of each of these elements is listed in the table below.

ElementPercentage Abundance of Element in Earth’s Continental Crust and in Sea
Phosphorus0.105
Arsenic0.00018
Antimony0.00002
Bismuth0.0000085

Phosphorous is the most abundant of group 15 elements found in Earth’s crust. Phosphorus can be found in many different minerals as part of a phosphate group (PO)43. An example of these so-called phosphate rocks is apatite, which is a phosphate mineral that has the formula Ca(PO)(F,Cl,OH)543 and is shown in the following figure.

An isolated sample of the mineral Apatite

Arsenic is a highly toxic element to humans due to the effect it can have on adenosine triphosphate (ATP). ATP is an energy-storage molecule that is present in all human cells. The breakdown of ATP releases energy that is used to carry out essential biological processes. Arsenic is toxic to humans because it disrupts the processes that produce ATP molecules. Without enough ATP, our cells cannot function and eventually this can be fatal.

Arsenic is also known to be a pollutant and to have carcinogenic properties.

Arsenic can be found naturally in its elemental form and as a mineral. Common arsenic-containing minerals include arsenopyrite (FeAsS) and rammelsbergite (NiAs)2. It can also be found in sulfide compounds such as AsS23 that is present in the mineral orpiment.

Not much antimony is present in Earth’s crust; however, it is found in more than 100 different mineral species. Stibnite is a soft gray sulfide mineral crystal that has the chemical formula SbS23. Stibnite is the most important source of antimony.

Finally, bismuth, an element that occurs roughly twice as much as gold, is most frequently found in bismuthinite (BiS)23 and bismite (BiO)23.

Group 15 elements are economically very important and have a wide number of uses. These broad applications stem partially from the fact that they are capable of existing in multiple oxidation states ranging from 3 to +5. For example, the elements of group 15 are capable of reacting with hydrogen to form hydrides such as ammonia (NH)3. In these hydrides, the group 15 element has an oxidation number of 3. Other examples of compounds of group 15 elements with varying oxidation states can be seen in the table below.

CompoundFormulaOxidation State
StibineSbH33
DiphosphanePH242
HydroxylamineNHOH21
PhosphorusP40
Hypophosphorous acidHPO32+1
Realgar (arsenic sulfide)AsS44+2
Bismuth oxideBiO23+3
Hypophosphoric acidHPO426+4
Antimony pentafluorideSbF5+5

Nitrogen itself can exist in an oxidation state ranging from 3 to +5. Most of these oxidation states occur in compounds containing hydrogen or oxygen, as shown in the table below.

CompoundFormulaOxidation State
AmmoniaNH33
HydrazineNH242
HydroxylamineNHOH21
Nitrogen, tetranitrogenN2, N40
Nitrous oxideNO2+1
Nitric oxideNO+2
Nitrogen trioxideNO23+3
Nitrogen dioxideNO2+4
Nitrogen pentoxideNO25+5

Atoms of phosphorus, arsenic, and antimony can independently bond together in different ways to make different types of allotropes.

Definition: Allotropes

Allotropes are different structural forms of the same element but in the same physical state.

Phosphorus has several different allotropes. White and red phosphorus are the most common allotropic forms, but violet and black allotropes also exist. White phosphorus is a waxy solid that has the chemical formula P4 and is composed of tetrameric molecules. The four phosphorus atoms are covalently bonded together, and they form a tetrahedral structure that is shown in the diagram below. The red, violet, and black allotropes have more complex molecular arrangements.

Arsenic has three common allotropes known as gray, yellow, and black arsenic. The gray form is known to be more common than the others.

Antimony is known to form one stable metallic allotrope and another three different metastable forms known as explosive, black, and yellow. The following figure shows the shiny metallic appearance of the most stable antimony allotrope.

99.99% fine antimony

Example 3: Selecting the Three Allotropic Forms of Arsenic from a List of Potential Allotropes

What are the three allotropic forms of arsenic?

  1. Red, black, and gray
  2. Red, black, and violet
  3. Blue, black, and red
  4. Violet, red, and gray
  5. Yellow, black, and gray

Answer

This question is testing our ability to recall facts regarding the allotropes of the different group 15 elements. Allotropes are different structural forms of the same element but in the same physical state.

While nitrogen and bismuth do not have any allotropic forms, arsenic, phosphorus, and antimony all have multiple allotropes. Phosphorus, antimony, and arsenic all have black allotropes.

Antimony and arsenic both have yellow allotropes, while the only element that has red and violet allotropes is phosphorus.

With this knowledge, we can, by process of elimination, correctly identify E as the correct answer: arsenic has allotropic forms of yellow, black, and gray.

Therefore, the correct answer is E.

The graph below shows the melting and boiling points for group 15 elements.

The melting points and boiling points tend to increase as we move down group 15; however, bismuth is a clear exception to this general rule. At atmospheric pressure, arsenic sublimes, changing directly from a solid to a gas. Nitrogen does not sublime, but the difference between its melting and boiling point is very small.

Some group 15 elements can form gaseous molecules, but they do not all have the same number of atoms. In the gas phase, nitrogen molecules usually contain just two atoms (N)2, although scientists have also managed to isolate the tetranitrogen molecule (N)4.

Phosphorus, arsenic, and antimony can exist as tetramer molecules in the gas phase, with the chemical formulas P4, As4, and Sb4. The tetramer molecules are stable at room temperature, but they start to decompose when the temperature gets too high to form diatomic molecules (P2, As2, or Sb2), much like nitrogen.

Bismuth can be considered to be quite similar to nitrogen in the sense that these elements can both form gases that are made up of diatomic molecules. This is unusual, as metals in the gas phase are often made up of just single atoms.

Group 15 elements all have five valence electrons, and this means they have many similar chemical properties. The electronic configurations of these elements are shown in the table below. We can see that group 15 elements all contain three valence electrons in a p subshell.

Atomic NumberElementElectrons per ShellElectronic Configuration
7Nitrogen2, 5[He]22sp
15Phosphorus2, 8, 5[Ne]33sp
33Arsenic2, 8, 18, 5[Ar]344dsp
51Antimony2, 8, 18, 18, 5[Kr]455dsp
83Bismuth2, 8, 18, 32, 18, 5[Xe]4566fdsp

Example 4: Identifying Similarities between the Characteristics of Arsenic and Antimony

Which of the following statements correctly states a similarity between arsenic and antimony?

  1. They are both more abundant in Earth’s crust than phosphorus.
  2. They are both metalloids.
  3. They both contain 4 electrons in their last p orbital.
  4. They are both less dense than nitrogen.
  5. They both form diatomic molecules under standard conditions.

Answer

In this question, we have to assess which statement correctly links arsenic and antimony.

Statement A describes the abundance of arsenic and antimony relative to that of phosphorus. The abundance of group 15 elements decreases as we go down the group, and so phosphorus is more abundant than both arsenic and antimony. Therefore, statement A is incorrect.

Statement B tells us that both elements are metalloids. When descending group 15, the characteristics of the elements change from nonmetals to metalloids and then to metals. Arsenic has the nonmetals nitrogen and phosphorus above it, and antimony has the metal bismuth below it. Statement B might be the correct answer.

Statement C tells us that they both contain four electrons in the last p orbital; however, all group 15 elements have five electrons in their valence shell, three of which are in the p orbital. So, statement C is incorrect.

Statement D describes both arsenic and antimony as being less dense than nitrogen. This is unlikely, as both arsenic and antimony are solids so are more dense than nitrogen, which is a gas.

Statement E suggests that both arsenic and antimony form diatomic molecules under standard conditions, similar to nitrogen. This is not true as they are both solids at room temperature.

Having assessed all the options, we can see that the correct answer is B.

Group 15 elements have the capacity to form many different types of oxide compounds because they can exist in various different oxidation states. Group 15 oxide compounds tend to be more basic when they contain atoms of group 15 elements that have higher atomic numbers. This can be understood by comparing dinitrogen pentoxide with antimony(III) oxide and bismuth(III) oxide:

  • Dinitrogen pentoxide (NO)25 is acidic.
  • Antimony(III) oxide (SbO)23 is amphoteric.
  • Bismuth(III) oxide (BiO)23 is basic.

The following table shows some of the other oxide molecules that can be formed by group 15 elements.

ElementFormula of Oxide
NitrogenNO2, NO, NO23, NO2, NO24, NO25, NO4, N(NO)23
PhosphorusPO23, PO25, PO47, PO48, PO49, PO, PO26
ArsenicAsO23, AsO24, AsO25
AntimonySbO24, SbO23, SbO25, SbO613
BismuthBiO23

Group 15 elements also have the capacity to form simple hydride molecules that have the chemical formula XH3, where X is a group 15 element atom.

We are probably already quite familiar with the hydride of nitrogen, otherwise known as ammonia (NH)3. However, arsenic and phosphorus can also form hydrides known as arsine (AsH)3 and phosphine (PH)3.

The solubility of group 15 hydrides in water tends to decrease with increasing atomic number. So, the solubility of ammonia in water is greater than that of phosphine, whereas stibine (SbH3) is only slightly soluble in water.

The following Lewis structure diagram shows how these hydrides tend to make three covalent bonds with hydrogen atoms and also have one lone pair of unbonded electrons.

NHHH

The electronegativity of group 15 elements decreases down the periodic table. Group 15 hydrides have less polar bonds and are less polar overall when they contain group 15 atoms with higher atomic numbers. The lighter group 15 hydrides such as ammonia are more soluble in water than heavier hydrides such as SbH3 and BiH3 because water is a polar solute. Molecules dissolve more readily in water when they are highly polar.

The thermal stability of group 15 hydrides decreases as we descend the group. Ammonia and phosphine are stable up until temperatures of several hundred degrees Celsius, whereas stibine is known to spontaneously decompose at room temperature.

Group 15 hydrides have different thermal stabilities because thermal stability depends on bond strength and the bond between an atom of hydrogen and the group 15 atom becomes weaker as the atomic radius of the group 15 atom increases.

The following equation shows how arsenic hydride molecules can break down and form hydrogen gas and arsenic products: 2AsH()3H()+2As()32ggs

Example 5: Understanding How the Physical Properties of Group 15 Elements Can Change as We Move down the Periodic Table

Which of the following properties is not true for group 15 elements?

  1. The melting points decrease down the group.
  2. The electronegativity decreases down the group.
  3. The metallic property increases down the group.
  4. The atomic radii increase down the group.
  5. The ionization energy decreases down the group.

Answer

This question is asking which of these properties is not true for group 15 elements.

Statement A tells us the melting point decreases down the group. However, if this were the case, then all of the other elements in group 15 would be gases in the same way as nitrogen. As this is not the case, then this choice may well be a correct answer, but we will eliminate the others to be sure.

Statement B says that electronegativity decreases down the group. We know that nitrogen is one of the most electronegative elements and that electronegativity increases across the periodic table from left to right and up the periodic table from the bottom. Therefore, as we descend group 15, electronegativity decreases, so statement B is true and, therefore, not the correct answer.

When moving down group 15 elements, their characteristics change from nonmetals to metalloids and then to metals. As a result, statement C is also true and, therefore, incorrect.

Statement D is also true, as the atomic radius increases with atomic number. Each element is in a different period of the periodic table, so the atomic number increases when descending the group.

Finally, the ionization energy also decreases down the group, as with increasing atomic radius and greater shielding, the valence electrons require less energy to be removed. We can, therefore, conclude that statement E is also true.

Statement A is the correct answer.

Having now assessed a broad range of different chemical and physical properties of group 15 elements, we can examine some of the common uses of these elements.

Nitrogen is important for producing ammonia, and ammonia is a critical component of many fertilizers. Fertilizers are used by different societies all over the world to improve the growth and output of different types of food crops.

Phosphorus is another important group 15 element because it can be used to make fertilizers. It can also be combined with other elements to make composite alloys that include materials such as phosphor bronze.

Arsenic is commonly used as a preservative for wood because its toxic nature can be exploited to reduce damage from insects, bacteria, and fungi.

Antimony can be used to produce different types of alloys and semiconductors, while bismuth can be used to make alloys when it is mixed with lead and cadmium.

The following table shows some other important uses of group 15 elements.

ElementUses and Applications
Nitrogen– It is used in the production of ammonia.
– It is used in the nitrogenous fertilizer industry.
– It provides an inert atmosphere for food such as potato chips.
– It is used in inflating race car and aircraft tires.
– It is used as a refrigerant for cryopreservation.
Phosphorus– It is used in manufacturing matches.
– It is used in the NPK fertilizer industry.
– It is used in alloys such as steel and phosphor bronze, which contains copper, tin, and phosphorus.
– Phosphoric acid is found in sodas.
Arsenic– It is a wood preservative.
– Arsenic trioxide is used as a cancer treatment.
– It is used in alloys with lead in the production of lead components for car batteries.
Antimony– Antimony trioxide is used as a flame retardant.
– It is used in the production of different alloys such as the lead–antimony alloy used in batteries.
– It is a stabilizer and catalyst for the production of the common plastic polyethylene terephthalate (PET).
– It is used in semiconductors for use in infrared detectors and diodes.
Bismuth– Bismuth subsalicylate is used as an antidiarrheal medication.
– It is used in alloys that can be used as solder due to its lower melting point.
– Bismuth vanadate is a yellow pigment.

Let’s summarize what we have learned about group 15 elements.

Key Points

  • Group 15 elements (also known as pnictogens) consist of nitrogen, phosphorus, arsenic, antimony, bismuth, and moscovium.
  • Group 15 elements have similar electron configurations, with each element having five valence electrons.
  • When descending group 15, the elements change from nonmetals to metalloids and then to metals.
  • Some group 15 elements, such as phosphorus, arsenic, and antimony, can form different allotropes.
  • The abundance of group 15 elements decreases when moving down the group.
  • Group 15 elements can form oxides, and these oxides tend to be more basic when they contain group 15 elements that have higher atomic numbers.
  • Group 15 elements can form hydrides, and solubility in water and thermodynamic stability tend to decrease when moving down the group.
  • Group 15 elements have many economically important applications, and these include the use of nitrogen and phosphorus elements to make fertilizers.

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